1. Field of the Invention
This invention relates to processing signals received from a communications channel. More particularly this invention relates to an integrated signal processing system for receiving signals suitable for use in the transmission of video, "hi-fi" audio, images or other high bit rate signals.
2. Description of the Related Art
Encoded transmission of inherently analog signals is increasingly practiced today as a result of advances in signal processing techniques that have increased the bit rate achievable in a channel. At the same time new data compression techniques have tended to reduce the bandwidth required to acceptably represent analog information. The art is presently striving to more efficiently transmit video and audio data in applications such as cable television using digital techniques.
Various modulation techniques have been employed in digital communications. For example quadrature amplitude modulation (QAM) is a relatively sophisticated technique favored by practitioners of digital radio communications. This method involves two separate symbol streams, each stream modulating one of two carriers in quadrature. This system achieves spectral efficiencies, between 5-7 bits/sec-Hz in multilevel formats such as 64-QAM and 256-QAM. QAM is particularly useful in applications having a low signal-to-noise ratio. However double sideband modulation is required. Furthermore cross-coupled channel equalizers are generally needed, which adds to the overall complexity of the system.
A variant of QAM is quadrature phase shift keying (QPSK), in which a signal constellation consisting of four symbols is transmitted, each having a different phase and a constant amplitude. The scheme is implemented as the sum of orthogonal components, represented by the equation. EQU A.sub.m .apprxeq.be.sup.j.theta..sbsp.m
where .theta..sub.m can be any of {0, .pi./2, .pi., 3.pi./2}. It is necessary to transmit both sidebands in order to preserve the quadrature information.
Another modulation scheme known to the art is vestigial sideband (VSB) modulation, which is achieved by amplitude modulating a pulsed baseband signal, and suppressing a redundant sideband of the amplitude modulated (AM) signal, in order to conserve bandwidth. Usually the lower sideband is suppressed. In the digital form of VSB, a digital pulse amplitude modulated (PAM) signal is employed.
It is proposed in Citta et al., Practical Implementation of a 43 mbit/Sec (8 bit/Hz) Digital Modem for Cable Television, 1993 NCTA Technical Papers, pp 271-278, to implement a 16 level VSB modulation method in cable television applications, wherein symbols of 16 discrete methods are amplitude modulated, using carrier suppression and transmission of a vestigial sideband in a 6 MHz channel. Transmission of a low level pilot carrier, located approximately 310 kHz above the lower channel edge, is included to assist in signal detection. The arrangement provides for the transmission of 43 Mbit/sec, but requires a passband of 5.38 MHZ at 4 bits/symbol.